I think BQ's tag is evidence that clues often sound more obvious to the person giving them than to the people reading them. N-Dubz? From Camden?

I didn't get that clue, either, perhaps BQ meant that their 'music' is somewhat like edge ware, but I wasn't about to try and find out.

I think the main reasons why this was a difficult tag was the way the picture was taken, as nothing useful came up with image search, and my aforementioned issue with the way it made the building appear, but that's a lesson learnt for me. Other than that, it was pretty clear from the clues, including the 1940 in the picture, that it was some kind of wartime installation and that there was an underground component to it. Then it was a bit of slogging through web-sites about that sort of thing, and there are only a few that come up easily when searching, so that made it a bit tricky. Good tag, though.

Ok, n dubs was simply the direction, in that it is a NW postcode.
All at sea was an admiralty reference, and the maps referred to the chart establishment.
Insurance policy was one of Oxgate Citadel’s code names, and deeper than that was a reference to the fact it disguised a bunker entrance.

Ha, I didn't pre tag it, but I did pass it, and went along a nearby road that must have been the source of its nickname. It took the whole day from Stratford with the (not so) little one in tow, and I foolishly went over the Heath past the Flask, (I'd have killed for a pint at that stage), so I probably should have banked a few while I was in the area. I desperately speed read the chapter of this area in John Rodger's This Other London before setting out looking for tag ideas! There's certainly one connected to Oxgate I'd like to go back to a little further out. Solo mission for that one.